We recommend using penny sleeves and semi-rigid holders. For your cards' safety, we recommend removing cards from magnetic, or one-touch holders. If the card is an extra thick card or a card placed in that particular holder by the manufacturer, we will accept them, but please keep in mind there is some risk in shipping cards in these types of holders.
CSG sells sleeves and semi-rigid holders, and the CSG ship kit comes with CSG semi rigid and CSG penny sleeves, to order the kit visit https://shop.collectiblesgroup.com/collections/csg-supplies.